Photochromic molecules are molecules that reversibly transform between two isomers with different absorption spectra while maintaining the same molecular weight when irradiated with appropriate wavelength of light. The diarylethene compound is known to exhibit excellent photochromic performance (Non-Patent Document 1). The diarylethene has a following structure, and it undergoes cyclization/cycoreversion reactions upon irradiation with light as shown in the following scheme.

Extensive studies have been carried out to apply the photochromic molecules as optical memory media that can optically store information (Patent Document 1, etc.). In such use, the media were prepared by dissolving photochromic molecules in an organic solvent, then spreading the resulting solution over a substrate.
Recently, bioimaging using fluorescent microscope, which is a method to observe an image by binding fluorochrome molecules to biomolecules, has been actively studied. Bioimaging that employs green fluorescent proteins (GFP) is frequently used, but this method is disadvantageous in that the label molecule is large, and the protein-protein interaction affects the target biomolecule. Diarylethenes are expected to achieve bioimaging with high resolution, since they are low molecular weight compounds. However, it is indispensable to provide water-solubility to the compounds for the application to biological samples.
In the studies of optical memory media mentioned above, the diarylethene compound did not need to be dissolved in water, so Patent Document 1 does not mention anything about a water-soluble diarylethene compound. Concerning water-solubility, Non-Patent Documents 3, 4 teach diarylethene compounds that have ionic groups or amphiphilic groups. However, these compounds tend to be aggregated in water, and would excessively affect the target molecule due to their strong ionic interaction, so they are hardly applied to bioimaging. Under such situation, a highly water-soluble diarylethene compound obtained by a different means was desired.